A system such as a radar uses an common antenna for the transmission and the reception. Such a system is designed such that a transmitter and receiver module including a high-frequency switch is defined by a transmitting circuit and a receiving circuit. Input and output terminals of the antenna, an input terminal of the receiving circuit, and an output terminal of the transmitting circuit are connected to one another by a circulator.
In the receiving circuit of the transmitter and receiver module described above, the high-frequency switch has its input terminal connected to the circulator. The high-frequency switch also has a first output terminal connected to an LNA (Low Noise Amplifier), and a second output terminal connected to a termination. At the time of reception of a signal, the high-frequency switch serves a function of allowing the signal to pass between the input terminal and the first output terminal, such that the received signal is output to the LNA at a stage following the high-frequency switch. At the time of transmission of a signal, the high-frequency switch serves a function of allowing the signal to pass between the input terminal and the second output terminal, such that the signal leaking via the circulator or the transmitted signal reflected from the antenna is output to the termination connected to the second output terminal, with the result that the LNA at the following stage is protected from the transmitted signal.
Further, power of a received signal input to the transmitter and receiver module significantly varies depending upon a distance between the radar and a target. When the power of the received signal is large, it is thought that an automatic gain control (hereinafter, “AGC”) circuit for adjusting power should be provided at a stage preceding the LNA or the high-frequency switch itself should have an AGC function in order to prevent the LNA from being broken down due to excessive power input to the LNA. When the high-frequency switch is used as the AGC circuit, the high-frequency switch is required to have a function of providing blockade between the input terminal and the first output terminal such that a received signal is sufficiently attenuated due to isolation of the high-frequency switch and is output to the LNA.
In a high-frequency switch having a circuit configuration illustrated in FIG. 1 of Patent Literature 1, all of the transistors are in an off-state when a signal is received, and all of the transistors are in an on-state when a signal is transmitted. Referring to FIG. 1 of Patent Literature 1, at the time of transmission of a signal, an on-resistance of a transistor 2c is small enough to be considered as being short-circuited when the high-frequency switch is viewed from an input/output terminal 1c connected to an LNA. As a result, total reflection from the high-frequency switch as the high-frequency switch is viewed from the input/output terminal 1c connected to the LNA occurs, which causes degradation of receiving characteristics or an unstable operation of the LNA. Accordingly, the high-frequency switch having the circuit configuration illustrated in FIG. 1 of Patent Literature 1 is difficult to use as an AGC, and a separate AGC circuit is required to be provided at a stage preceding the LNA.